Re: Should ARP modify the cache...?

[Athena <linux@xxxxxxxxxxxx>]

On Thu, 7 Sep 2000, M K Saravanan wrote:

> 
> Should ARP (Addr. Resolution Protocol) Modify the cache even when it
> receives (ARP request for some other machine) informtion without
> specifically requesting it?
> 
> ** mks **
> 
        There are 2 cases


1. A machine say A gets an ARP request for its IP address (see Note 1),
then it adds the hardware/IP address of the sender to its ARP cache. Logic
is somebody is going to send me packet and I will be replying to it. So it
avoids an extra ARP (later when the application will reply to the
request -- ARP is not required for unicast ARP reply, as I have the
hardware address in the request,  the question is whether or not to cache
it)

2. a. A machine receives an ARP request for some other IP address and it
has an entry in the ARP cache for the sender (the person who sends the
request) IP address. Then the ARP cache is updated.
   b. A machine receives an ARP request for some other IP address and it
does not have an entry in ARP cache. In that case a new ARP CACHE entry is
not added.(Logic is you may not be receiving any request from that
machine, why waste ARP cache entry)



Note 1:
   ARP request is a broadcast at the ethernet level (ethernet address is
set to ff.ff.ff.ff.ff.ff

Note 2:
   ARP reply is unicast.

Note 3:
   A complete ARP cache entry is valid for only 20 seconds.(unless you
enter the entry manually and make it permanent) and after that it is
deleted.
   Incomplete ARP cache entry remains in cache for only 3 minutes.
   

George Samuel
























































  






































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